A Python error is not just a failure. It is a clue about what your program tried to do and why it could not do it. Learning to read that clue is one of the most useful programming skills you can develop.
Start with the last line of the traceback
When Python raises an exception, the traceback shows the sequence of calls that led to the problem. The last line usually names the exception and provides a short explanation.
Read the error message first, then locate the relevant line in your own code. The first line in the traceback is not necessarily where the mistake occurred.
Example: understanding a TypeError
Consider a program that tries to add a number to a string. Python does not automatically know whether you want arithmetic or text concatenation.
age = 20
message = "Age: " + age
# TypeError: can only concatenate str (not "int") to strFix the underlying cause
If you want to build a text message, convert the number to a string or use an f-string. If you want to perform arithmetic, check that both values are numbers.
age = 20
message = f"Age: {age}"
print(message)A repeatable debugging checklist
Work through these checks in order instead of randomly changing lines until the error disappears.
- Read the exception type and message.
- Find the line in your code identified by the traceback.
- Inspect the values and types used on that line.
- Create the smallest example that reproduces the error.
- Fix the cause and rerun the program.
- Test the corrected code with another input.
When to get another explanation
If you understand the error message but not the programming concept behind it, revisit the concept with a smaller example. A tutor or knowledgeable reviewer can help you understand why the fix works, not just what to change.
Understanding the concept is the first step.
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